1. Field of the Invention:
The conventional radio data communication terminal shown in FIG. 5 includes A/D converters 41 and 42 for A/D-converting I (in-phase), Q (quadrature) signals which have been orthogonally demodulated in an RF unit (not shown). The terminal further includes a phase rotator 43 for rotating the phase of the received signals by a desired phase detection circuit 44 for detecting phase difference between the present angle and an angle before one period. The terminal further includes a average value calculating circuit 45 for calculating an average value by summing the phase differences detected by phase difference detection circuit 44 by a predetermined number of times and dividing the summation by the predetermined number and the number of symbols in the period. The terminal further includes a integration circuit 46 for integrating the average detected by average value calculating circuit 45 every symbol time. The terminal further includes a vector resolving circuit 47 for resolving the integrated value supplied from integration circuit 46 to a real part and an imaginary part and outputting them to phase rotator 43. The terminal further includes a transmission path distortion estimator 48 for estimating the transmission path distortion during the preamble period by using a pair of signals whose phase has been rotated by phase rotator 43. The terminal further includes a tap coefficients setter 49 for calculating tap coefficients necessary for equalizer 50 on the basis of the transmission path distortion determined by transmission path distortion estimator 48 and setting the coefficients to equalizer 50. The terminal further includes a equalizer 50 for equalizing the pair of signals from phase rotator 43 on the basis of the tap coefficient set by tap coefficient setter 49 to perform a demodulation.
2. Description of the Prior Art:
Among radio data communication terminals which operate in accordance with the narrow-band modulation system such as GMSK modulation system, there are such terminals that detect a frequency offset and execute training for estimating a transmission path distortion during a preamble period.
FIG. 5 shows the construction of a conventional radio data communication terminal.
The conventional radio data communication terminal shown in FIG. 5 includes A/D converters 41 and 42 for A/D-converting I (in-phase), Q (quadrature) signals which have been orthogonally demodulated in an RF unit (not shown), phase rotator 43 for rotating the phase of the received signals by a desired phase, phase difference detection circuit 44 for detecting phase difference between the present angle and an angle before one period, average value calculating circuit 45 for calculating an average value by summing the phase differences detected by phase difference detection circuit 44 by a predetermined number of times and dividing the summation by the predetermined number and the number of symbols in the period, integration circuit 46 for integrating the average detected by average value calculating circuit 45 every symbol time, vector resolving circuit 47 for resolving the integrated value supplied from integration circuit 46 to a real part and an imaginary part and outputting them to phase rotator 43, a transmission path distortion estimator 48 for estimating the transmission path distortion during the preamble period by using a pair of signals whose phase has been rotated by phase rotator 43, tap coefficients setter 49 for calculating tap coefficients necessary for equalizer 50 on the basis of the transmission path distortion determined by transmission path distortion estimator 48 and setting the coefficients to equalizer 50, and equalizer 50 for equalizing the pair of signals from phase rotator 43 on the basis of the tap coefficients set by tap coefficient setter 49 to perform a demodulation.
At a transmission side, the same PN sequence (pseudo noise sequence) is repetitively transmitted during a preamble period inserted before an information data period. At a radio data communication terminal, phase difference detection circuit 44 detects the phase difference between samples which are apart from each other by the period of the PN sequence during a predetermined period within the preamble period. Average value calculating circuit 45 divides the phase difference between the samples which are apart from each other by the period of the PN sequence by the number of symbols during one period of the PN sequence to calculate a symbol-based phase difference xcex94xcex8 (frequency offset value), and hold this phase difference xcex94xcex8 during one frame period comprising the preamble period and the information data period. Integration circuit 46 integrates the phase difference xcex94xcex8 on a symbol basis, and vector resolving circuit 47 resolves the output of integration circuit 46 to vector components. Phase rotator 43 corrects the phase at the receiver side by using the output of vector resolving circuit 47. After the phase difference xcex94xcex8 is calculated, transmission path distortion estimator 48 estimates the transmission path distortion, and tap coefficient setter 49 calculates the tap coefficients necessary for equalizer 50 on the basis of the transmission path distortion thus determined and sets the tap coefficients to equalizer 50.
Furthermore, according to another prior art disclosed in JPA-6-252698, an automatic adaptive equalizing means repetitively reads a training signal from storage means until tap coefficients or an impulse response converges to the transfer function of a transmission path, whereby the training sequence can be shortened. Therefore, the training signal to be added before the data signal in the same frame of the signal transmitted from the transmission side can be shortened to enhance the information transfer efficiency.
In the prior art shown in FIG. 5, it is necessary to successively perform the detection of the frequency offset value, the estimation of the transmission path distortion, the setting of the tap coefficients to the equalizer, etc. in this order within the preamble period before the equalizer is actuated, and thus the preamble period is lengthened. Therefore, this technique has a disadvantage that the information transfer efficiency is reduced.
Further, in the prior art disclosed in JPA-6-252698, when the same training signal is repetitively read out from the storage means in the process of converging the tap coefficients or the impulse response to the transfer function of the transmission path, there is a disadvantage that the processing time of the operation become increased.
In order to overcome the aforementioned disadvantages, the present invention has been made and accordingly has an object to provide radio data communication terminal of a narrow-band modulation type which can determine a frequency offset value used for phase rotation means and set tap coefficients used in an equalizer by using the shortest preamble.
According to the present invention, there is provided a radio data communication terminal, which includes a phase rotating means for correcting a phase error of a frame signal which conforms to a narrow-band modulation system. The terminal further includes an equalizer for correcting a transmission path distortion of the frame signal corrected in phase. The terminal further includes a calculating means for calculating the phase error by using a preamble in the frame signal, wherein the phase error is used for the phase error correction in the phase rotating means. The terminal further includes a transmission path distortion estimating means for estimating the transmission path distortion by using the preamble in the frame signal corrected in phase and calculating tap coefficients necessary for the equalizer. The terminal further includes a setting means for setting the tap coefficients to the equalizer. The terminal further includes a first delay means for delaying the frame signal. The terminal further includes a selection means for selecting an input or output of the first delay means and outputting the selected one to the phase rotating means so that the calculating means and the transmission path distortion estimating means operate while using the same portion of the preamble.
The radio data communication terminal may further comprise second delay means for further delaying the output of the first delay means, and the selection means may further select an output of the second delay means so that information data in the frame signal is input to the phase rotating means after the setting operation of the setting means is completed.
In the ratio data communication terminal, the calculating means may comprise: phase difference detection means for detecting a phase difference between two symbols in the preamble, wherein the two symbols are apart from each other by a predetermined period; average value calculating means for calculating an average phase difference per symbol as a frequency offset; integrating means for integrating the frequency offset every symbol; and vector resolving means for resolving an output of the integrating means into a real part and an imaginary part.
In the radio data communication terminal, the narrow-band modulation system may be a GMSK modulation system.
According to the present invention, a preamble period can be shortened because the same portion of a preamble is used for calculation of a frequency offset and estimation of a transmission path distortion.